The present invention relates, in general, to imaging systems for low level radiation sources where high levels of extraneous wide-band light may be present in the field-of-view and, in particular, to Laser Warning Receivers (LWR) for protecting military platforms against laser guided weapons by detecting and locating very low power laser sources associated with those weapons when light from other sources, such as the sun, may be present in the same field-of-view.
Many research laboratories and manufacturers around the world are presently involved in the development of Laser Warning Receivers (LWR) for the purpose of protecting military platforms against laser guided weapons by detecting, identifying and locating laser sources associated with those weapons. One type of LWR is described in U.S. Pat. No. 5,428,215 by J. Dubois et al. The detection threshold of most LWRs are limited, however, by extraneous light from other sources and particularly by the high level of background radiation generated by the sun. The sun is almost always the limiting factor for highly sensitive light detection systems required to detect and locate very low power laser sources such as associated with a Laser Beam Rider (LBR) where the laser is aimed at a target to guide a missile towards that target. The detectors for these types of very low power laser sources still have a limited capability because the signal they produce is often buried in the background radiation. It is possible to reject part of the sun""s radiation through optical filtering but this is at the expense of a narrower optical bandwidth covered by that LWR. Even when using optical filters, the ultimate limit in the detection threshold is the residual sun radiation.
The sun is also a problem for other imaging systems, such as video cameras when the sun is in the field-of-view of the image sensor. This results in it being very difficult to balance the brightness of the image and a lot of information in the image is lost because of the very bright spot of light generated by the sun at one location in that image.
Various techniques already exist for the detection of laser sources, some of which use narrow-band optical filters, multiple narrow field-of-view (FOV) detectors or multiple lenses combined with a detector array to handle the problem of radiation from the sun during the detection of such sources. LWRs based on these types of approaches succeed in reducing the effects of the sun""s radiation but, most of the time, this leads to very costly and bulky devices. One disadvantage of using multiple lenses combined with a detector array is that a large number of detection channels result. The use of filters has limitations due to the large number of them required for a successful rejection of radiation from the sun.
It is an object of the present invention to provide a light detection system capable of discriminating between high levels of radiation from bright extraneous sources and light from other sources and of attenuating the high levels to allow the detection system to detect the light from other sources.
An imaging system for a field-of-view where high levels of light from an extraneous source may be present at one area in the field-of-view, according to one embodiment of the present invention, has means to focus an image of the field-of-view onto a surface of a photodetector, an optical limiter being located between the means to focus and the photodetector, the optical limiter having means to limit transmission of said high levels of wide-band light received from the extraneous source at a particular area where the high levels of light are directed onto the optical limiter by the means to focus, the optical limiter remaining transmissive to light received from areas in the field-of-view outside of said particular area allowing light received from outside of said one area to be focused onto the photodetector.
A light detection system for detecting laser beams where high levels of wide-band light from an extraneous source may be present at one area in the system""s field-of-view, according to a further embodiment of the invention, has means to focus an image of the field-of-view onto a surface of a photodetector, an optical limiter being located between the means to focus and the photodetector, the optical limiter having means to limit transmission of said high levels of wide-band light received from the extraneous source at a particular area where the high levels are directed onto the optical limiter by the means to focus, the optical limiter remaining transmissive to light received from areas in the field-of-view outside of said particular area allowing any laser beam received from the field-of-view to be focussed onto the photodetector.